PROJECT ABSTRACT Although hepatotropic viruses are important causes of human disease, the intrahepatic immune response to hepatitis viruses is poorly understood due to a lack of tractable small animal models. In this Multi-PI application, an experienced virologist teams with an accomplished viral immunologist to propose experiments aimed at better understanding the intrahepatic immune response to a human hepatitis virus using a novel murine model in which hepatitis A virus (HAV) infection of mice lacking the type I interferon (IFN) receptor (Ifnar1-/-) recapitulates critical features of type A hepatitis in humans. Preliminary data from this model demonstrate (1) that the capacity of HAV to evade MAVS-mediated type I interferon (IFN?/?) responses defines its host species range, and (2) that acute HAV-induced liver injury stems from IFN-independent intrinsic hepatocellular apoptosis with secondary inflammation that unexpectedly results from MAVS and IRF3/7 signaling. This new murine model thus reveals a previously undefined link between innate immune responses to virus infection and acute liver injury, providing a novel paradigm for viral pathogenesis in the liver that is likely relevant to other hepatitis viruses. The HAV 3ABC protease cleaves human MAVS, disrupting IFN responses in human cells, but cannot cleave the murine MAVS ortholog due to sequence differences. We hypothesize that this inability of HAV to disrupt murine MAVS-mediated IFN responses renders mice nonpermissive for infection, and will test this hypothesis in Aim 1 using mice with CRISPR/Cas9 editing of the Mavs gene that renders the expressed protein susceptible to 3ABC cleavage. Aim 1 will also define cell types that sense HAV and in which IFN-induced gene expression controls HAV replication in the liver, and ascertain the role played by type III IFN? in countering infection in the liver and intestinal mucosa. Aim 2 will define differences in protective, antiviral IFN-induced transcriptional responses versus pro-pathogenic IRF3-induced gene expression, examine the role of IFIT proteins in the pro-pathogenic response, and determine whether transcription-independent functions of activated IRF3 control infection or drive acute liver injury. Aim 3 will examine the adaptive cellular immune response to HAV infection and how it is linked to the early innate immune response. Aim 3 will address the role of HAV-specific T cells in virus control and define innate mechanisms regulating these responses in the liver. The proposed studies will further refine a unique and valuable animal model and provide novel insight into the interplay between innate and adaptive immunity and acute liver injury in the context of hepatotropic virus infection.